Both my MacBook Pro and my 20″ iMac are running Leopard 10.5.8. On both machines, it’s easy to turn off the Airport from the command line:

sudo ifconfig en1 down

On the MacBook Pro, it’s also easy to turn the Airport back on in the obvious way:

sudo ifconfig en1 up

But on the iMac, nothing happens if I issue that command. I don’t know why, but tonight I discovered how to get the job done:

sudo networksetup -setairportpower On

Why did I care, you might ask? Until 10.5.8, the iMac wouldn’t reliably join the network after the machine woke from sleep (or rebooted), and I had to turn the Airport off and on — I could turn it off easily enough from the command line, but I had to use the GUI to turn it back on, which seemed silly. Now I can do the whole thing from the command line (but with luck, I won’t have to).